Irradiation with high energy ions for surface structuring and treatment of surface proximal sections of optical elements

ABSTRACT

A method for processing the surface of a component, or the processing of an optical element through an ion beam, directed onto the surface to be processed, so that the surface is lowered and/or removed at least partially, wherein the ions have a kinetic energy of 100 keV or more, as well as optical elements processed by the method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for surface structuring of acomponent through an ion beam, and to a method for treating an opticalelement, in particular an optical element for an optical system inmicrolithography applications, with an ion beam as well.

2. Prior Art

In the state of the art different methods for treating materials andcomponents with ion beams are known. Thus, it is known, e.g. to usefocused ion beams (FIB) for imaging and manipulating surfaces. For thesemethods, acceleration voltages for ions, like e.g. gallium in the rangeof 5 to 50 kV, and corresponding currents of 2 pA to 20 nA are beingused. The ion beam can be focused with electrostatic lenses to adiameter of a few of nm, and can then be guided over the surface, lineby line, through respective deflection.

Through the interaction of the ion beam with the surface, so-calledsputter processes (atomization processes) occur, which lead to thepossibility of a treatment of the materials in the nm range.

However, this method cannot be used for topography corrections ofoptical elements, due to the direct removal of the surface, since, dueto a local use of this method, also the micro-roughness is locallychanged.

Furthermore, it is known e.g. to use ion beam methods with loweracceleration energies, this means ions with energies in the range of 0.2keV to 1.2 keV for treating surfaces of optical elements, like e.g.lenses for objectives in microlithography applications. Herein, a loweracceleration voltage is being used, compared to the focused ion beammethod, so that only a lower removal occurs directly in a layer of 1 to2 nm from the surface. Thereby, it can be accomplished, that themicro-roughness of the surface is maintained, and only larger sizetopography errors are corrected. However, this method has lowerefficiency, due to the lower removal rate. Furthermore, in thecorrection of topographic errors, in the range of <1 mm, there areproblems with positioning precision, since ions in this energy range arehard to focus.

Furthermore, also high energy ion beam methods are known, in which ionsare implanted in components or materials with acceleration energies ofup to 3 MeV, or more. This method of ion implantation is mostly used fordoting semiconductors.

From DE 41 36 511 C2 a method for producing a Si/FeSi₂-heterostructureis known, wherein iron ions are implanted into a silicon substrate withthe iron ions being irradiated with an energy of 20 keV to 20 MeV ontothe substrate.

DE 38 41 352 A1 discloses e.g. the implantation of bor, carbon,nitrogen, silicon or hydrogen ions in a silicon-carbide layer during theproduction of a silicon carbide diaphragm for a radiation lithographymask. Herein ion implantations serve the purpose to achieve a stressrelaxation and better optical transparency in an oxide layer formedduring subsequent temperature treatment.

The U.S. Pat. No. 4,840,816 describes doping of crystalline oxides likeLiNbO₃ with heavy metals for forming a beam waveguide. The ions areimplanted with a doping density in the range of 1.2×10¹⁷ to 2.5×10¹⁷ions per cm² with energies of about 360 keV at a temperature of −190° C.

Due to these different areas of application, the basics of theinteraction of ion beams with materials have already been researchedintensively. From this research it is known that the ions are sloweddown, when impacting the material, through various braking mechanisms,like inelastic collisions with bound electrons, inelastic collisionswith atom nuclei, elastic collisions with bound electrons, and elasticcollisions with atom nuclei etc. An overview of the resultingmacroscopic and microscopic effects in amorphous silicon dioxide isgiven e.g. in the publication by R. A. B. Devine in “Nuclear Instrumentsand Methods in Physics Research” B91 (1994) pages 378 to 390.

DISCLOSURE OF THE INVENTION Object of the Invention

It is an object of the invention to provide a method, allowing a surfacestructuring in the sense of a at least partial subsidence, and/orremoval of the surface of a component with greater efficiency,resolution, and/or precision, than the currently known methods, whereinin particular the micro-roughness of the surface shall be maintained. Arespective method shall be useable in particular for treating opticalelements, like optical lenses or mirrors for optical systems inmicrolithography applications. Thus, in particular it shall not causeany undesired changes of the material, or of the optical element withthis respect. Furthermore, an exact local treatment shall be possible.

Technical Solution

This object may be accomplished through methods or optical elements asparticularized in the claims. Advantageous embodiments are subjectmatter of the dependent claims.

The invention is based on the inventors having found out as a surprisethat when using high-energy ions, thus energies above 100 keV, through achange of the prevailing braking mechanism of the ions directly at thesurface, no higher locally acting energies are available, than in thecurrently used method in the range of 1 keV. Accordingly, there arehardly any restructurings, or direct material removals directly at thesurface. Thus, there is also no change in micro-roughness. In addition,coatings like anti-reflection coatings or reflection coatings beingpresent at the optical surface are not influenced. Furthermore, mostlyinelastic electron excitations (electronic stopping) can be observed inthis energy range, and no elastic particle collisions (nuclear stopping)can be observed, so that far reaching changes in the material, whichcould lead to undesired and uncontrollable changes, when used in opticalelements, were avoided. However, a subsidence, and thus structuring ofthe surface can be accomplished through a compaction or volumereduction.

At the same time, the high acceleration energies ≧100 keV, in particular≧200 keV, and preferably ≧400 keV, however provide the advantage that avery good beam guidance and focusing, and a very good positioningprecision of the beam are possible.

Thus, a very effective processing or correction of topographic errors inthe sub millimeter range can be accomplished.

The energy range of the ions can be in particular 500 keV to 5000 keV or600 keV to 2000 keV, which, on the one hand assures that a respectivelydesired surface subsidence and/or a removal with simultaneously smallfurther changes of the material occurs, and furthermore, a good handlingof the ion beam with respect to the local positioning and focusing isfacilitated.

Thus, preferably a processing and treatment of optical elements, likee.g. optical lenses from silica, fused silica, on silica based glass orULE (ultra low expansion)-materials can be performed. Glass ceramicmaterial, like Zerodur can also be processed. Altogether, processing ofall materials is possible which are transparent or reflective at thewavelengths of the electromagnetic irradiation used for lithography andwhich do not undergo undesired changes of properties through processing.

Accordingly, every material, which can particularly be applied asrefractive or diffractive material at wavelengths of 365 nm, 248 nm, 193nm or 158 nm or as reflective material for EUV (extreme ultraviolet)radiation with wavelengths at 13.5 nm, may be used as material to beprocessed. Accordingly, reflective optical elements like mirrors and thematerials correspondingly used therefor can also be processed. Inparticular, it is possible to process optical elements or opticalsurfaces, on which a coating is already provided, like e.g. refractiveand diffractive optical elements with anti-reflection coatings orreflective optical elements with reflection coatings. Since the ionswith the respective energies are slowed down in an area below thesurface and lead to corresponding structural changes, which contributeto subsidence in the irradiated area, a corresponding coating can bemaintained at the optical surface without any damages. Altogether,already finished, usable optical elements can be processed or can becorrected afterwards.

Only an increase of the refraction index in an area below the surface,which corresponds to the braking range of the ions, can be observed.This increase of the refraction index can be compensated through arespective optical layout of the optical system, and can thus even beused for targeted production of optical elements. In such a casepossibly the correction of the surface or topography cannot be the primeconcern, but the targeted adjustment of the optical properties, or ofthe refractive index in certain ranges of the optical element.Accordingly, this is an aspect of the invention, for which independentprotection is claimed.

For example, ions having different energies can be subsequentlyirradiated into the same surface area so that the depth of the impactarea is different. Thus, areas having a different refraction index canbe generated e.g. in the direction of the irradiation direction startingfrom the surface. For instance, a near-surface layer of silica or fusedsilica may have a refraction index of e.g. 1.5 at a wavelength of theused electromagnetic irradiation of 193 nm, while in the subsequentbraking area of the high-energy ions, the refraction index is raised toa value of 1.6 to 1.7 and in the subsequent area again the originalvalue of the respective material of 1.5 is present. In a further,lower-lying layer ions with higher energy as those, which contributed tothe changes of the first layer area with amended refraction index, mayagain generate an area with elevated refraction index in the range of1.6 to 1.7 so that a layered structure of alternating low and highrefraction index layers is obtained. Thus, a layer stack of thin layershaving different refraction indices may be produced, which can be usedas reflection structure, for example.

Through the missing interaction of the ion beam with the materialdirectly at the surface, the micro-roughness of the surface can bemaintained. In particular, in a range of 0.05 to 0.2 nm RMS (root meansquare) at a local wavelength of 10 nm to 100 μm, the micro-roughnesscan be maintained in spite of the surface treatment.

The surface removal with direct removal of material is negligible at therespective energies. The surface subsidence with volume change in adepth area of 100 nm from the surface is however accomplished inparticular through a change of the material structure in the brakingrange of the ions. It depends on the energy of the impacting ions and onthe fluence, this means the imparting ions per surface area. However,saturation can occur beyond a certain fluence. The change of thematerial structure can be caused e.g. in the case of silica materialthrough a change of the average 12-ring tetraeder structure into anaverage 3 to 4-ring tetraeder structure.

The beam current of the ion beam can assume values of 1 to 100 nA,preferably 5 to 25 nA, and in particular 10 nA. The value of 10 nAcorresponds in Si-ions approximately to 6×10¹¹ ions per second. Thus,depending on the irradiation time, fluences, i.e. impacting particlesper surface area, in the magnitude of 10¹³ to 10¹⁶ ions per cm² arereached.

The ion beam may have a diameter of 1 to 5000 μm, in particular 10 to2000 μm, especially 50 to 200 μm, wherein a positioning with any desiredprecision is possible. However, a positioning in the magnitude of atenth of the beam diameter is sufficient for the present application, sothat the ion beam can be positioned with an accuracy of 0.1 to 50 μm, inparticular 1 to 20 μm.

During the process, the ion beam can be moved over the surface to betreated, wherein the beam is deflected by respective electrical and/ormagnetic components, and guided over the surface to be treated.Alternatively, also the piece of material can also be moved relative tothe ion beam, wherein the deflection of the ion beam is preferred incase of a locally fixed surface, which is to be treated, due to thepositioning precisions which can be reached.

The processing or treatment can be performed in a lateral range of up toseveral 100 mm. The lower limit is thus given by the beam diameter andpreferably amounts to 10 μm.

Different species can be used as ions, wherein ions of noble or inertgases like elements of the 8^(th) main group of the periodic table withhelium, neon, argon, krypton, xenon, radon or ununoctium as well asnitrogen or oxygen are advantageous. Further, ions of elements orcompounds, which are included in the material to be processed, can beused. In particular, silicon ions can be used for fused silica, or glassceramic materials, like Zerodur, which are used in optical elements, inparticular refractive or diffractive optical elements in objectives formicrolithography applications, since they can be integrated into thenetwork of the material, without leading to far-reaching changes of thematerials properties due to being a foreign material. In general,non-metals or non-semi-metals are therefore preferred, in particular asthey are contained in the optical element.

As already mentioned above, only an increase of the refraction indexthrough compacting the material in the area of the braking range of theions is observed, wherein the refraction index can be continuouslyincreased, in case of fused silica from 1.5 to 1.6 to 1.7 for awavelength of the light used of 193 nm. This effect, however can also beadvantageously used in case of optical elements to produce structureswith different refraction indices, which is also an aspect of thisinvention.

In general, a continous increase of the refraction index can be achievedfor the corresponding optical materials like fused silica, silica,silica-based glass, ultra-low expansion (ULE)-material or glass-ceramicmaterial like zerodur in the magnitude of 0 to 20%, particularly 5 to15% for the wavelength range in the magnitude between 150 nm and 350 nm,particularly in the wavelength range of 190 nm to 250 nm which is thewavelength range which is interesting for refractive or diffractiveoptical elements in microlithography.

Processing of the surface of a respective optical element or componentwith the ion beam can be carried out at ambient temperature in thetemperature range of 0° C. to 50° C., preferably 15° C. to 30° C. andparticularly at room temperature.

After the ion beam treatment, the treated component can be exposed to atemperature treatment, and in particular to a temperature treatment atpreferably more than 200° C., in particular 300° C. or more, for aduration of several hours, and in particular 24 hours, so that possiblestructural damages can be healed. However, the accomplished geometriceffects, like e.g. subsidence, and/or removal of the surface and thecompacting of the material, are maintained in the braking area.

Accordingly, optical elements can be produced by means of the presentinventive method, namely in particular optical lenses or mirrors,wherein the optical elements comprise a near-surface area which isspaced to the surface, i.e. is located with a distance to the surface,with the near-surface area having an increased refraction index comparedto the surrounding areas without amendment of the material composition,i.e. for areas having identical chemical compositions. For example, anisle-like area with increased refraction index can be achieved, whichserves for correction of aberrations in an optical system of amicro-lithography installation, like an illumination system orprojection objective. Furthermore, it is also possible to provide alayer with increased refraction index over the whole optical surface ofthe optical element or at least in a partial area thereof. The layer canbe positioned in an area of 100 nm to 5 μm beneath the surface dependingon the chosen energy of the ions.

According to a multiple treatment of the same surface area with ions ofdifferent energies, areas with different refraction indices may beproduced, with the areas of different refraction indices succeeding eachother in depth direction. The areas with different refraction indicesmay adjoin each other or may be separated by areas having a lowerrefraction index. The latter is a preferred structure in a directionacross the surface, particularly perpendicular to the surface, withsubsequent and alternating areas with increased and lower refractionindex. Such a structure can be used as a reflector for electromagneticirradiation and may be adapted according to the wavelength of theelectromagnetic irradiation with respect to the thicknesses of thedifferent areas.

The areas with increased refraction index may correspond with surfacesections which are lowered with respect to surrounding surface sectionsso that according to the near-surface structure with differentrefraction indices at the same time a structure with respect totopography is present. For example, this is given, when the inventivemethod is carried out at an originally flat surface. However, the methodcan also be used with an already structured surface, i.e. a surfacehaving a topography formation so that after carrying out of the methodthe different refraction indices are present in near-surface areas, butthe surface being again flat or flush, respectively.

BRIEF DESCRIPTION OF THE FIGURES

Further advantages, characteristics and features become apparent duringthe subsequent detailed description of an embodiment, based on theappended drawings. The figures show in a purely schematic manner in:

FIG. 1 the illustration of the principle of a device, which can be usedfor the method according to the invention;

FIG. 2 a depiction of the mode of operation of the method.

FIG. 3 a sectional view across a surface of the first optical element;and in

FIG. 4 a sectional view across the surface of a second optical element.

PREFERRED EMBODIMENT

FIG. 1 shows a device for performing the method according to theinvention in a schematic illustration. In an ion source 1, ions aregenerated, which are accelerated towards an aperture 2, through arespective voltage applied by a voltage source 6. Through ion beamoptics 3, which are made from suitable electrical and/or magneticcomponents, the ion beam 5 can be focused. The focused ion beam can bedeflected into two different directions, which are illustrated by thedouble arrows, through a deflection unit 4, which in turn comprisesrespective electrical and/or mechanical components. Accordingly, the ionbeam 5 can be guided in a scanning manner over the component to beworked or treated, wherein the ions there interact with the material ofthe component 7 to be processed.

The generation of the ions in the ion source 1, and a possibleextraction of the ions through an electrostatic field, or the separationof the ions, corresponding to their mass in a magnetic field, can beperformed according to known methods and is not illustrated anddescribed here in more detail.

According to a preferred embodiment, a device illustrated in FIG. 1 wasused in order to irradiate silicon ions with energies in the range of500 to 2000 keV onto silica. With 700 keV Si-ions, the range of the ionsin the material amounted to approximately 1 μm, wherein the maximumrange depends on the energy of the ions used, with E^(2/3). The physicalmaterial removal during an irradiation with 10¹⁶ ions per cm² amounts to1 nm, while the effective surface subsidence amounts to several 10 nmthrough a change of the material structure in the braking range of theions.

Through the compaction in the braking range of the silicon ions, anincrease of the refraction index to values of 1.6 to 1.7 occurs.

Subsequent to the treatment with the ion beam, there is a temperaturetreatment at 300° C. for 24 hours. Through the temperature treatment,the accomplished geometric effects with respect to the subsidence of thesurface and compaction in the subsequent area are not changed. However,there is a healing of other disorders, since the absorption maxima foundbefore the tempering at wavelengths of 216 nm, 243 nm, and 280 nm,cannot be formed anymore after the temperature treatment. A change ofthe transmission at a wavelength of 193 nm cannot be detected throughthe entire treatment of the silica.

FIG. 2 shows the use of the method according to the invention, and theinteraction of the ion beams with the material to be treated in the formof two schematic images a) and b). In the left partial image of FIG. 2a), the surface 8 of the part 7 to be treated is shown with a surfaceroughness 12 in the form of a surface elevation. Through the treatmentof the area of the surface 8, which is associated with the surfaceelevation 12 through ion beams 5, according to the presented method, dueto the high energy of the ions >100 keV or >500 keV in the selectedembodiment a penetration of the ions occurs in a section 10 below thesurface 8 of the material. In the penetration section 10, the ions areincreasingly slowed down through inelastic particle collisions, so thata change of the structure of the material and a compacting occursthrough repositioning into an energetically more favorable state.

Furthermore, in particular cases, also a removal of material occurs fromthe work piece 7, wherein the removed material, as indicated by thearrows 9, does not originate from an area directly at the surface 8, butfrom areas located below. At the surface 8, there is no directinteraction with the high energy ions of the ion beam 5, since thekinetic energy of the ions in this area is too high. Thus, the surfacestructure is also not changed directly, this means the surface roughnessis not changed and is maintained, as also the partial image b) of theFIG. 2 shows for the state after the treatment.

Through the compaction of the braking area 10 through the irradiatedhigh energy ions, thus after the treatment a respectively compactedrange 11 under the surface 8′ is present, wherein the surface elevation12′ is removed. In the compacted area 11, there is an increasedrefraction index of the treated silica in the range of 1.6 to 1.7.

FIG. 3 shows a sectional view across the surface of an optical elementwhich was treated with the method according to the present invention.The surface comprises sections 80 which are untreated, as well assections 81 which were irradiated with corresponding ions. Accordingly,below sections 81 in which treatment of the optical element with highenergy ion beams took place, compacted areas 110 can be found whichcomprise an increased refraction index, for example. For two compactedareas 110 compactions are associated with subsidence of the surfacesections 81 with respect to the surrounding, non-treated sections 80.For the compacted area 110 at the left side of FIG. 3, the correspondingsurface section 81 is disposed in the same plane as the adjoining,untreated section 80. This is achieved, when, as shown in FIG. 2, theoriginal surface sections was elevated with respect to the surroundingsections so that by compaction of the underlying material a leveling hastaken place.

FIG. 4 shows in a further sectional view across the surface area of anoptical element the formation a layer structure comprising areas withnone-increased refraction index 113, 114 and areas with increasedrefraction index 111, 112.

The untreated material 115 may e.g. have a refraction index of 1.5 atwavelengths of the used light of 193 nm. Due to subsequent orsimultaneous irradiation with ions of different energy leading to apenetration of ions in differently deep areas 111 and 112, thecorresponding material is compacted and an increase of the refractionindex in the areas 111 and 112 is created. Between the areas 111 and 112having increased refraction indices an area 114 may be present, as shownin FIG. 2 in which the originally lower refraction index is present.However, the energy of the ions may also be such that the areas withincreased refraction index 111, 112 lie adjacent to each other or mergecontinuously. Due to use of high energy ions having a minimum energy anunchanged area 113 directly in the neighborhood to the surface 180remains.

Through a structure as shown in FIG. 4 a reflactor can be formed inwhich the refraction indices of the areas 111, 112, 113, 114 and 115 aswell as the corresponding thicknesses are set such that light ofspecific wavelength is reflected.

Though the present invention has been described in detail with referenceto a preferred embodiment, it is appreciated by a person skilled in theart that variations and changes, in particular through a differentcombination of the described features of the invention, and also theomission of particular features are possible, without departing from thescope of the appended claims.

1. A method for processing the surface of a component, comprisingirradiating a surface by an ion beam directed onto the surface to beprocessed, so that the surface is lowered at least partially, whereinthe ions comprise a kinetic energy of at least 100 keV.
 2. A method forprocessing an optical element, comprising irradiating a surface by anion beam, directed onto the optical element to be processed, wherein theions comprise a kinetic energy of at least 100 keV.
 3. A methodaccording to claim 1 or 2, wherein the ions have a kinetic energy of atleast 200 keV.
 4. A method according to claim 1 or 2, wherein the ionshave a kinetic energy of at least 400 keV.
 5. A method according toclaim 1 or 2, wherein the ions have a kinetic energy of 500 keV to 2000keV.
 6. A method according to claim 1 or 2, wherein the processing iscarried out at an optically effective, usable surface of an opticalcomponent.
 7. A method according to claim 1 or 2, wherein processing iscarried out at a surface provided with a coating.
 8. A method accordingto claim 7, wherein the coating is at least one of an anti-reflectionlayer or a reflection layer.
 9. A method according to claim 1 or 2,wherein structuring is achieved with respect to at least one of alateral extension of the surface to be processed and a depth direction.10. A method according to claim 1 or 2, wherein the same surface area istreated with ions of different energies in at least one manner out of agroup comprising a treatment in subsequent time periods and treatment atthe same time.
 11. A method according to claim 1 or 2, wherein duringprocessing micro-roughness is maintained
 12. A method according to claim1 or 2, wherein during processing micro-roughness is maintained in therange of 0.05 to 0.20 nm RMS (root means square) at a local wavelengthof 10 nm to 100 μm.
 13. A method according to claim 1 or 2, whereinthrough at least one of the setting of the energy of the ions and of thefluence, at least one of the amount of surface subsidence and removal isadjusted.
 14. A method according to claim 1 or 2, wherein the beamcurrent is 1 to 100 nA.
 15. A method according to claim 1 or 2, whereinthe beam current is 5 to 25 nA.
 16. A method according to claim 1 or 2,wherein the beam current is approximately 10 nA.
 17. A method accordingto claim 1 or 2, wherein the fluence is 10¹³ to 10¹⁶ ions/cm².
 18. Amethod according to claim 1 or 2, wherein the ion beam has a diameter of1 to 5000 μm.
 19. A method according to claim 1 or 2, wherein the ionbeam has a diameter of 10 to 2000 μm.
 20. A method according to claim 1or 2, wherein the ion beam has a diameter of 50 to 200 μm.
 21. A methodaccording to claim 1 or 2, wherein the ion beam can be positioned with aprecision of 0.1 to 50 μm
 22. A method according to claim 1 or 2,wherein the ion beam can be positioned with a precision of 1 to 20 μm.23. A method according to claim 1 or 2, wherein the ion beam is movedover the surface to be processed, with the surface to be processed beingfixed in place and the ion beam being deflected.
 24. A method accordingto claim 1 or 2, wherein the treatment is performed laterally in a rangeof 10 μm up to several 100 mm.
 25. A method according to claim 1 or 2,wherein ions are used out of a group comprising ions of noble and inertgases, ions which are comprised in the material to be treated, non-metalions, non-semi-metal ions and Si-ions.
 26. A method according to claim 1or 2, wherein at least one out of a group comprising diffractive opticalelements, refractive optical elements, reflective optical elements,optical lenses and optical mirrors is processed.
 27. A method accordingto claim 1 or 2, wherein the refraction index of a processed element isincreased in the braking range of the ions.
 28. A method according toclaim 27, wherein the refraction index in fused silica is increased to1.6 to 1.7.
 29. A method according to claim 1 or 2, wherein therefraction index is increased by 0 to 20%.
 30. A method according toclaim 1 or 2, wherein the refraction index is increased by 5 to 15%. 31.A method according to claim 1 or 2, wherein the ion beam is irradiatedto the surface to be processed at an ambient temperature of 0 to 50° C.32. A method according to claim 1 or 2, wherein the ion beam isirradiated to the surface to be processed at an ambient temperature of15° C. to 30° C.
 33. A method according to claim 1 or 2, wherein thecomponent to be processed is exposed to a temperature treatment aftertreatment with the ion beam for a duration of at least one hour.
 34. Amethod according to claim 1 or 2, wherein the component to be processedis exposed to a temperature treatment after treatment with the ion beamat a temperature of at least 200° C. for a duration of at least onehour.
 35. A method according to claim 1 or 2, wherein the component tobe processed is exposed to a temperature treatment after treatment withthe ion beam, at a temperature of at least 300° C. for a duration of atleast one hour.
 36. A method according to claim 1 or 2, wherein thecomponent to be processed is exposed to a temperature treatment aftertreatment with the ion beam for a duration of at least 24 hours.
 37. Amethod according to claim 1 or 2 wherein the surface is at leastpartially removed.
 38. Optical element for microlithography comprising anear-surface area which is at a distance to the surface, wherein thearea has an increased refraction index compared to the surrounding area.39. Optical element according to claim 38, wherein plural areas withincreased refraction index are disposed adjacent to each other in adirection across to the surface.
 40. Optical element according to claim38, wherein plural areas with increased refraction index are disposedsubsequent to each other and alternating with areas of lower refractionindex in a direction across to the surface.
 41. Optical elementaccording to claim 38, wherein the areas with increased refraction indexare disposed beneath a surface section which is at the same level as thesurrounding sections.
 42. Optical element according to claim 38, whereinthe areas with increased refraction index are disposed beneath a surfacesection which is lowered compared to surrounding sections.
 43. Opticalelement according to claim 38, wherein the areas with increasedrefraction index and the surrounding areas of lower refraction indexdiffer mainly in the structure and not in the composition.
 44. Opticalelement according to claim 38, wherein the refraction index of the areawith increased refraction index is increased by 2 to 20% with respect tothe areas with lower refraction index.
 45. Optical element according toclaim 38, wherein the refraction index in the areas with increasedrefraction index is increased with respect to the areas with lowerrefraction index by 5 to 15%.
 46. Optical element according to claim 38,wherein the refraction index in the areas with increased refractionindex is increased with respect to the areas with lower refraction indexby 7 to 10%.
 47. Optical element according to claim 38, wherein theoptical element comprises silica and an area with a refraction index of1.5 and areas with a refraction index of 1.6 to 1.7 at wavelengths of193 nm.
 48. Optical element according to claim 38, wherein thenear-surface area is extending into a depth of up to 5 μm from thesurface.
 49. Optical element according to claim 38, wherein the opticalelement comprises regions having different compositions.
 50. Opticalelement according to claim 38, wherein the region comprises a surfacecoating.
 51. Optical element according to claim 50, wherein the surfacecoating comprises at least on of an anti-reflection coating and areflection coating.